Most people who engage in unprotected sex don't think they'll get an STD. Based on their impression of their sexual partner, cursory risk assessment of the activity, and their level of intoxication, most people assume they'd be able to tell if their partner had an STD before engaging in sexual activity. There is no real consensus about what signs would clue you in to whether or not someone has one of the burning, drippy, bumpy, or worse— silent but deadly— infections that can be spread by intimate contact. So, how can you tell if someone has an STD?